Even in the best electric machines conversion losses occur resulting in heat that has to be transferred out of the electric machine to avoid overheating and/or damage.
The term ‘electric machine’ comprises all types of electric motors that convert electric energy into mechanical energy, electric generators for generating electric energy and/or electric converters that convert electric energy (AC or DC) with a certain voltage and/or frequency into an other form of electric energy (AC or DC) with other voltage and/or other frequency.
All these machines have in common that high currents occur and as a result of these high currents and the electric resistance of the active components of the electric machines heat occurs inside the active components. This heat has to be transferred out of the electric machine to avoid overheating of the machine.
Electric machines are often cooled by means of cooling air that flows through the machine. This cooling air absorbs heat from the active components (for example the rotor and/or the stator of a generator or a motor and the semiconductors of an electric converter) and heats up.
Often cooling air is forced through the electric machine by means of a closed cooling air circuit. One advantage of a closed circuit is that the cooling air is always clean and no humidity or dust enters the machine. To transfer the heat from cooling air to the ambient, a heat exchanger is installed inside the electric machine as a part of the closed cooling air circuit. This results in an enlarged space demand of the cooling air circuit and consequently the size of the electric machine increases. In addition, the provision of a heat exchanger also requires additional pumps for circulating the coolant. These additional pumps are extra components that are prone to failure. In other words, the provision of additional pumps may impair the reliability of the apparatus.
From DE 10 2007 021 723 A1 a rotating electric machine comprising a closed air cooling circuit is known. This cooling air circuit comprises heat exchangers inside a housing of the electric machine to cool down the cooling air.